1. Field of the Invention
The present invention relates to a threaded joint for joining tubes used, in particular, in the natural-gas and oil extraction industry, which has the characteristics of claim 1. Said tubes can be used both as pipes for pumping gas or oil, and as casings of the drilling wells themselves.
2. Description of the State of the Art
In oilfields it is common practice to use metal pipes of a definite length, joined to form a string such as to reach the depth at which the reservoirs of gas or oil are found for extraction of the latter.
This extraction technique calls for the gradual internal coating of the well, in the course of drilling, using metal tubes, the so-called “casing”. Once the desired depth has been reached, a string of metal tubes of smaller diameter, the so-called “tubing”, is inserted in the “casing”, for pumping the liquid or gas hydrocarbons to the surface. Both the column for the casing and the column for the tubing are made up of pipe segments assembled by means of threaded joints of a male and female type, so-called pin and box members.
To enable the operation of assembly it is common practice to use a lubricant to facilitate joining of the connectors. The lubricant normally used contains within it small particles of heavy metals, such as lead or copper. These heavy metals are dangerous for health and for the environment.
Apart from the risk of pollution, another problem that must be tackled is the determination of the quantity of lubricant, since performance of the joints is very sensitive to the variations of said quantity.
In general, galling occurs since an amount of lubricant less than the amount necessary does not ensure correct lubrication and does not enable the high contact pressure generated by the interference between the male and female threads to be sustained.
A problem opposite to the one described above is encountered in the use of dope and is the so-called “over-doping”, or excess of dope, which entails the difficulty, during screwing of a joint, of disposing of the excess dope at the ends of the threads of the segments of tube. The entrapped dope can, as a result of this, develop high pressures in the joint. In particular circumstances, such pressures are able to produce a plastic deformation of the pipe segments in the threaded portion, and may lead to the collapse of the pin member. Once said phenomenon has occurred, the joint loses its effectiveness, and the pipe segments and/or the sleeve must be replaced.
Entrapping occurs when the dope does not have sufficient space for penetrating the interstices of the thread, or else if there is not sufficient space for it to flow on the outside of the connection.
The above problem is even more serious in the case of a joint with more than one seal. In this case, the lubricant is trapped between two seals, is not able to flow away, and it is impossible to reduce the high pressure that is developed.
A possible solution to the above problem is to provide, as reservoir for the lubricant, a void space obtained between the two seals.
The disadvantage resulting from providing a reservoir of lubricant consists in the creation of a considerable void space between the seals, which generates a loss of continuity and can thus reduce the performance of the joint.
Even if the lubricant is applied in a correct amount, the same problems referred to above may arise, mainly if there is a lack of uniformity of distribution over the entire area of the joint.
In this regard, it must be considered that the operation of application of the lubricant is carried out on the platform at the moment in which the string is lowered into the well and is consequently particularly expensive in terms of time and resources. The frequently adverse environmental conditions render both the precise dosage of the amount of lubricant and its application on the threaded parts problematical.
In order to reach the ever greater depths at which accumulations of hydrocarbons are found, the levels of performance required by the market are continuously increasing, as demonstrated by the increase in the thickness of the walls in use for oil extraction.
In order to deal with the above problems, there have been proposed solutions regarding the dosage and sizing of the spaces that are to be occupied by the lubricant.
A solution to the above problem of entrapment of lubricant has been proposed in the U.S. Pat. No. 4,830,411, which fixes the minimum extension of the void space between the male thread and the female thread. Said value is fixed at 0.6 mm2/pitch. In said empty space the lubricant can expand and prevent the problems of imprisonment.
A further way of improving the performance of the joints, principally when the pipe works in compression, consists in reducing the gap between the acute side of the male and female threads, as disclosed in patent document WO 0066928, which suggests a value for said thread gap of 0.002 ins. (corresponding to 0.05 mm).
As a result of the above, considering, for example, a threaded joint with 6 TPI (threads per inch), with a height of each thread of 1 mm, the empty space above the male thread and the female thread, according to the teaching of U.S. Pat. No. 4,830,411, must be at least 0.26 mm in order to prevent problems of pressure of the lubricant. Thus the above total empty space represents a difference of diameter of 0.52 mm, between the outside of the male tube and the inside of the female tube.
The area of the critical section of a joint is the area of an annulus, and said area is different for the pin and for the box. For the pin said annulus is defined by the inner diameter (ID) of the pipe, on the one hand, and by the diameter at the groove of the last thread engaged, whilst for the box said annulus is defined by the outer diameter (OD) of the pipe and by the diameter at the groove of the last thread engaged.
Consequently, if we consider the quantities OD and ID of the tubes to be constant, said difference of 0.52 mm between the outside of the pin and the inside of the box can reduce the performance of the joint.
Furthermore, on account of the large gap on the stabbing sides of the thread, the threaded portion does not work when compressive loads are applied.
It emerges therefrom that preventing the problems of pressure of the lubricant and achieving high performance in the joint are requirements that go in opposite directions.